1. Field of the Invention
The present invention relates to a piston with a ring groove that has fitted thereinto a piston ring structural body wherein a ring continuously formed of resin without being provided with an abutment is pressed radially outwards by a coil expander composed of a metal wire wound like a coil.
2. Description of the Related Art
To reduce the amount of blow-by gas (gas which leak to a crank case through a gap between a piston ring and a cylinder), Japanese Patent Application Laid-Open No. HEI 9-280373 discloses a piston equipped with a piston ring continuously formed of resin without being provided with an abutment. This publication discloses a piston ring structural body wherein an outer ring continuously formed of polytetrafluoroethylene (PTFE) as a fluororesin without being provided with an abutment is pressed radially outwards by a ring tensile force applying member composed of a metal coil expander via an inner ring also continuously formed of PTFE without being provided with an abutment.
In the piston ring structural body disclosed in the aforementioned publication, the ring is divided into the inner and outer rings. While the inner ring ensures sealability around a ring groove, the outer ring ensures sealability between a sliding portion thereof and a cylinder wall.
If the piston ring is provided with an abutment, a small amount of combustion gas leaks out toward a crank case through the abutment. The leaking combustion gas blows down oil which has adhered to the ring groove of the piston and the cylinder wall toward the crank case, whereby the amount of oil consumption is reduced.
However, in the case of a ring made of resin without being provided with an abutment, the aforementioned blow-down effect cannot be obtained. As a result, the oil that has adhered to the ring groove of the piston and the cylinder wall is not returned toward the crank case, and the amount of oil consumption may increase.